Introduce Options and use them for Menu.

Introduce Options for components. This allows me to add new features,
without updating functions signatures.

include/ftxui/util/ref.hpp

@@ -0,0 +1,63 @@
+#ifndef FTXUI_UTIL_REF_HPP
+#define FTXUI_UTIL_REF_HPP
+
+#include <ftxui/screen/string.hpp>
+#include <string>
+
+namespace ftxui {
+
+// An adapter for a const object referenced or owned.
+template <typename T>
+class ConstRef {
+ public:
+  ConstRef() {}
+  ConstRef(T t): owned_(t) {}
+  ConstRef(const T* t) : address_(t) {}
+  const T& operator*() { return address_ ? *address_ : owned_; }
+  const T* operator->() { return address_ ? address_ : &owned_; }
+
+ private:
+  T owned_;
+  const T* address_ = nullptr;
+};
+
+/// @brief For convenience, this class convert multiple mutable string
+/// references toward a shared representation.
+class StringRef {
+ public:
+  StringRef(std::wstring* ref) : address_(ref) {}
+  StringRef(std::wstring ref) : owned_(std::move(ref)) {}
+  StringRef(const wchar_t* ref) : StringRef(std::wstring(ref)) {}
+  StringRef(const char* ref) : StringRef(to_wstring(std::string(ref))) {}
+  std::wstring& operator*() { return address_ ? *address_ : owned_; }
+  std::wstring* operator->() { return address_ ? address_ : &owned_; }
+
+ private:
+  std::wstring owned_;
+  std::wstring* address_ = nullptr;
+};
+
+/// @brief For convenience, this class convert multiple immutable string
+/// references toward  shared representation.
+class ConstStringRef {
+ public:
+  ConstStringRef(const std::wstring* ref) : address_(ref) {}
+  ConstStringRef(std::wstring ref) : owned_(std::move(ref)) {}
+  ConstStringRef(const wchar_t* ref) : ConstStringRef(std::wstring(ref)) {}
+  ConstStringRef(const char* ref)
+      : ConstStringRef(to_wstring(std::string(ref))) {}
+  const std::wstring& operator*() { return address_ ? *address_ : owned_; }
+  const std::wstring* operator->() { return address_ ? address_ : &owned_; }
+
+ private:
+  const std::wstring owned_;
+  const std::wstring* address_ = nullptr;
+};
+
+}  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_UTIL_REF_HPP */
+
+// Copyright 2020 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.